PNNL

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are common air pollutants and combustion byproducts that exhibit diverse mutagenic, carcinogenic, proinflammatory and teratogenic properties. Oxygen-substituted PAHs (OPAHs), like the multi-ringed unsubstituted (parent) PAHs, are formed during combustion, as well as via phototoxidation and biological degradation of parent PAHs. Despite their prevalence both in contaminated industrial sites and in urban air, OPAHs mechanisms of action in biological systems are relatively understudied. Like parent PAHs, OPAHs exhibit structure-dependent mutagenic activities and differential activation of the aryl hydrocarbon receptor (AHR). In the canonical AHR signaling pathway, the AHR translocates to the nucleus upon ligand binding, dimerizes with the aryl hydrocarbon receptor nuclear translocator (ARNT), and binds to DNA response elements to activate transcription of a suite of downstream genes that include cytochrome p450 phase 1 metabolizing enzymes (CYP1A, CYP1B1). Four-ring OPAHs 1,9-benz-10-anthrone (BEZO) and benz(a)anthracene-7,12-dione (7,12-B[a]AQ) were found to cause morphological aberrations and markers of oxidative stress in developing zebrafish with similar potency, but only 7,12-B[a]AQ induced robust Cyp1a protein expression. We investigated the role of the AHR in mediating the toxicity of BEZO and 7,12-B[a]AQ, and found that knockdown of ahr2 rescued developmental effects caused by both compounds. Using comparative mRNA-seq, we show that BEZO is a less potent inducer of phase 1 metabolism gene expression than 7,12-B[a]AQ, while genes that regulate redox-homeostasis were affected similarly by these two OPAHs. BEZO uniquely decreased expression of genes that mediate eye development. Biological functions and upstream regulators associated with the significantly altered transcripts suggest that BEZO is a weaker AHR agonist than 7,12-B[a]AQ, but interacts with other transcriptional regulators to cause developmental toxicity in an AHR-dependent manner. Identifying ligand-dependent AHR interactions and signaling pathways is essential for understanding toxicity of this class of environmentally relevant compounds.